Objects under a water surface, i.e. submarine objects, can coarsely be detected from a position above the water surface using a sound navigation and ranging (sonar) device.
Sound navigation and ranging devices are based on transmitting an acoustic wave in the water towards the object, and detecting an acoustic wave which has been reflected by the object. The acoustic wave can be pulsed and/or modulated. Common modulation schemes comprise e.g. a continuous transmission frequency modulation (CTFM) scheme.
The attainable precision and resolution of object detection are mainly influenced by the properties of the applied acoustic wave such as the center frequency or the covered frequency bandwidth of the acoustic wave. Furthermore, the propagation scenario of the acoustic wave in the water, in particular attenuation and propagation speed, influences the attainable precision and resolution of object detection.
Common sound navigation and ranging devices for the detection of objects under a water surface operate in an acoustic frequency range extending from 10 kHz to 1 MHz. The propagation scenario is non-stationary due to pressure variations and consequently propagation speed variations of the water medium and poses significant challenges to object detection. Undesired reflections of the acoustic wave at the water surface and the water ground further impede the detection of objects.
Therefore, the detection of objects under a water surface using a sound navigation and ranging device is limited to the detection of large objects at short or medium ranges. The detection of small objects or at long ranges is usually not possible due to the non-stationary propagation scenario and the limited frequency bandwidth of the acoustic wave.